The present invention relates in general to CDMA (Code Division Multiple Access) and in particular to methods and systems for interleaving data in a unique manner before transmission over multiple diversity paths to increase system capacity without compromising reliability of signal reception.
CDMA (Code Division Multiple Access) technology, in cellular communication systems, involves the use of different codes to distinguish different user communications rather than different frequencies as commonly used in the initial cellular systems. The codes utilized are referred to as Walsh codes. While the number of codes is finite, the number of simultaneously occurring communications is typically limited by available power, rather than the number of codes available. This is primarily the limitation on capacity and is due to the forward link or downlinkxe2x80x94the communication path between base stations (BTS) and mobile station (MS). The reverse link or uplinkxe2x80x94communication path between mobile station and base stationxe2x80x94capacity is generally limited by interference. However, in deployed CDMA networks, the forward link capacity is the limiting factor in determining number of simultaneous communications that can be served with a given grade of service (GOS). This forward link capacity with respect to a given signal, is primarily a direct function of the magnitude of power required to provide satisfactory reception of all other communication in the vicinity. If all the signals in a cell can be transmitted at a lower power and still be satisfactorily received, there is more available power for new users and thus a potential for increased system communication capacity.
SNR (signal-to-noise ratio) is a term used to express the power of a signal relative to noise (interference). This ratio is generally expressed in dB (decibels) and is a logarithmic function. A common term in CDMA is Eb/No (bit energy to total noise power spectral density) and is used to define the strength of the traffic signal received by an MS relative to the noise or interference from other sources. Two principal sources of interference are Ioc (total received interference power from cellular communications in adjacent cells) and Ior (total received power from cellular communications originating within a cell of interest). A further energy parameter used in the CDMA art is Ec where Ec denotes energy per chip. Ec,pilot refers to the pilot channel while Ec,traffic refers to the traffic channel. A related but different SNR represented as Ec/Ior is used to represent chip energy received from the base station relative to the total received power from a given cell. Therefore, Ec,traffic/Ior is the percentage of power required for the traffic channel. Another concept in cellular technology is signal diversity. With appropriate equipment, the informational content for a sub-set of the informational content of a signal may transmitted on more than one frequency, or from one or more antennas, or at different times and so forth to mitigate the effects of signal fading. General information on these forms of diversity can be obtained from any digital communications textbook. The specific names are frequency diversity, spatial diversity, time delay diversity, and so forth. The receiving equipment may utilize the additive combination of the multiple received signals in the detection process. In theory, and usually in practice, fast fading is uncorrelated across the different diversity branches and does not occur on all of the received signals at the same time and thus the receiver typically has enough signal strength to correctly decode the received data. Slow signal fading may occur due to atmospheric conditions but more often, in cellular technology, it is due to interference in the form of physical obstructions between the base station transmitter and a given MS receiver. Hence, transmit diversity may not be able to offer any diversity over the slow fading. Fast fading is due to the way sub paths of signals add constructively and destructively as the MS moves. The sub paths are caused by reflections from objects near the receiver, and are offset in time and phase relative to the other sub paths. The duration of fast signal fading typically is reduced as the movement velocity of the MS increases.
The term transmitter diversity is typically defined as a technique whereby an information sequence is transmitted from more than one diversity branch. This can take the form of multiple frequencies or multiple antennas spaced effectively to mitigate the effects of signal fading. In this document, specifically, we use transmit diversity in the form of spatial diversity (i.e. multiple antennas).
The spatial diversity technique used is to explain the usefulness of the invention. This specific example does not limit the scope of the invention. The invention is also applicable to other forms of transmit diversity, such as frequency diversityxe2x80x94multiple diversity frequency branchesxe2x80x94and so forth. These transmissions are designed to ensure independent fading on the different signal paths. Proper combining of the paths at the receiver reduces the severity of the fading. In general, transmit diversity can be subdivided into two main classes, feed-forward diversity schemes and feedback diversity schemes. In feedback methods, measurements made by the mobile and transmitted to the network allow base stations to adjust the transmissions to make optimal use of the different transmission paths. Feedback techniques provide the potential for more performance improvement than feed-forward methods, at the expense of greater complexity.
Several transmit diversity schemes have been considered for inclusion in a new version of CDMA often referred to as cdma2000 or 3G (third generation). The combination of transmit diversity and fast power control is expected to yield a forward link capacity double that of previous IS-95 specification compliant systems. It is important to determine which of the many possible diversity techniques provides the greatest capacity increase at the lowest cost to the overall system design.
The term xe2x80x9cinterleavingxe2x80x9d, as used in this document, refers to a communication technique, normally used in conjunction with error correcting codes, to reduce the number of uncorrected bit error bursts. In the interleaving process, code symbols are reordered before transmission. One subset of this definition is that they are reordered in such a manner that any two successive data bits or code symbols are separated by Ixe2x88x921 symbols (or bits) in the transmitted sequence, where I is called the degree of interleaving. Another subset of this definition is that they are reordered such that all originally consecutively occurring bits or symbols are maximally separated. As will be apparent, many other reordering subsets may be generated. The equipment or software for accomplishing this interleaving in a given communication channel is designated as a xe2x80x9cchannel interleaverxe2x80x9d.
It is known that the specific design of channel interleavers for transmit diversity schemes may affect system capacity in a cellular system. By system capacity, we mean the total number of MSs that can simultaneously operate in a given cell where the forward link capacity is the limiting capacity factor, and is dependent on the required transmit power per user.
In the cdma2000 proposed standard, a feed-forward transmit diversity scheme is designated as OTD (Orthogonal Transmit Diversity). The originally proposed method of OTD involves encoding and interleaving an information bit stream into a coded bit stream. The streams are then de-multiplexed into two separate streams in a round-robin fashion for transmission over two spatially separated antennas. Each separate stream is then mapped into Quadrature Phase-Shift Keyed (QPSK) symbols and spread by different Walsh codes orthogonal to one another. The spread sequences are then scrambled by a quadrature pseudo-noise (PN) sequence, which is the same from all users of the same sectors. Output streams from each sector are mutually orthogonal, and therefore same-cell interference is eliminated in flat fading channels. By splitting the coded data into two or more data streams, the effective number of spreading codes per user is the same as the case without OTD. Different orthogonal pilots are used and transmitted over the different antennas. This allows coherent detection of the signals received from both antennas.
At the mobile receiver, RAKE fingers demodulate the two parallel paths separately. The matched filter demodulator used is called a RAKE correlator because of the resemblance of the tapped-delay-line matched filter to an ordinary garden rake. That is, the RAKE matched filter/correlator resembles a garden rake in the way it collects the signal energy from all the resolvable multipath components. For more details of the characteristics and performance of a RAKE correlator, reference may be made to Proakis (1989) and to the original works of Price (1954, 1956) and Price and Green (1958).
The two data streams are demodulated using two different sets of Walsh codes. The mobile multiplexes the two paths before de-interleaving and decoding. After the information from each stream is demodulated, the two data streams are multiplexed. The multiplexed stream of soft symbols is used by the mobile to estimate the received Eb/No, which is in turn used to trigger fast power control commands to the serving base stations. One Eb/No estimate is made each power control group, and this estimate is compared to a threshold to determine the value of the power control bit. This is similar to the method used to compute fast reverse link power control commands at the base station in IS-95 networks. Only the power control bits (for reverse link power control) that are punctured onto the forward link data stream are used to compute the Eb/No estimates. The power of the traffic channel bits is dependent on the frame sub-rate, and therefore, a priori knowledge of the frame sub-rate is required in order for all traffic channel bits to be used in the Eb/No estimates. Therefore, using punctured power control bits, which are always sent at full rate, circumvents this problem.
The prior art interleaving schemes have been found to have serious capacity limitations, at low MS moving velocities and in particular when used with OTD. The capacity limitation at low MS moving velocities is also applicable to the above mentioned OTD as proposed for cdma2000. While the originally occurring consecutive bits were separated in time of transmission, blocks of originally consecutive 16 bits were to be transmitted from the same antenna due to the interleaving methodology utilized. At lower mobile speeds, fading may easily span time comparable to the duration of a frame of data. The loss of such a quantity of consecutively occurring bits prevents the decoder from easily reconstructing the original data; thus increasing the Eb/No requirements to meet the given GOS. This limitation is also applicable to other diversity techniques that send a sub-set of information on different diversity branches. Examples of these other techniques include TSTD (Time Switched Transmit Diversity), STD (Selective Transmit Diversity), multi carrier (multiple frequency diversityxe2x80x94where a subset of the data bits are sent on one frequency, and another subset sent on a different frequency) and so forth.
It is desirable to have an interleaving methodology that improves the signal decoding performance of receiving equipment at low MS velocities. It is also desirable that such a methodology provides an improved SNR, since this consequently increases system capacity.
The present invention comprises an improved interleaving methodology for use with path diversity radio transmissions wherein each path comprises a portion of a communication message.